Hydraulic cylinders of the type disclosed in U.S. Pat. No. 3,848,880 generally utilize a seal assembly on the piston thereof to prevent leakage of fluid past the piston. As taught in said patent, a unitary composite seal assembly for such cylinders comprises a relatively hard synthetic rubber U-cup and a relatively softer synthetic rubber sealing ring that fills a central recess in the U-cup. The sealing ring may be mechanically retained or bonded within the U-cup.